This invention relates generally to resistive temperature devices (RTDs) and more specifically concerns a RTD measuring module which is part of a specific system arrangement involving a RTD, an RTD measuring module and a remote device such as a protective relay, which uses the RTD information from the RTD measuring module.
A resistive temperature device (RTD) is a resistor which has a positive temperature coefficient (i.e. the resistance of an RTD increases as the external temperature increases and decreases as the temperature decreases). The variation in resistance for a given RTD is known over a selected range of temperatures. An accompanying device, usually referred to as a sensing or measuring module, is used to determine the external temperature (the temperature in the immediate vicinity of the RTD) by first measuring the RTD resistance and then determining the temperature using a lookup table or similar technique which associates temperature and resistance for that particular category of RTD, based on its particular temperature coefficient. Thus, the RTDs and their associated measuring modules are used when it is desirable to accurately measure temperature at a particular location.
There are many applications for RTD devices, including the determination of ambient temperature at locations where the reading of a conventional temperature gauge is impossible or inconvenient, as well as determining change in temperature in concealed parts of certain apparatus, such as transformers, generators and motors. The outputs from the RTD measuring modules can be used to control the operation of such apparatus, and various processes as well.
In motor protection, for instance, RTDs are used to detect the operating temperatures of the motor bearings and the stator windings as well as the temperature of the incoming (cooling) air. It is important to know such operating temperatures to prevent overheating of the apparatus. Different types of RTDs are used for particular applications. Several different types of RTDs may be used in a single apparatus to monitor the temperatures of different parts of the apparatus. The different types of RTDs are discussed in more detail below.
RTD measuring circuits (modules) are often present in protective relays which are used to protect the operation of various apparatus. When the measuring module recognizes that the temperature in a particular RTD location is out of range, it will change the operation of the apparatus (such as by reducing speed) or temporarily stop operation by means of a circuit breaker.
In a typical case, the protective relay is located a substantial distance (hundreds of feet in some cases) from the protected apparatus; a long, shielded, multiconductor cable is required between the RTDs and the RTD measuring circuitry (module) in the relay. Such an arrangement increases the installation cost of the RTD system and creates a slight risk of protective relay damage in the event of winding insulation failure in the protected apparatus. Attempting to locate the protective relay closer to the protected apparatus to reduce such cost is often difficult to accomplish, while locating the RTD measuring module separate from the relay creates significant communication and RTD setting burdens associated with the measuring module.
Hence, it is desirable to reduce the cost of the connection between the measuring module and the RTDs in the protected apparatus without changing the location of the protective relay.
Accordingly, the present invention is a setting-free temperature-measuring module apparatus for a first plurality of resistive temperature devices (RTDs) connected thereto, comprising: means for measuring the resistance of said first plurality of RTDs present at selected sites where temperature is to be monitored; means for storing the resistance/temperature maps for a selected number of different types of RTDs, which includes at least those types comprising said first plurality of RTDs; processing means for determining temperature values associated with said resistances from the resistance/temperature maps stored in the module; and means for transmitting all the determined temperatures for said first plurality of RTDs to a remote device which determines the correct temperature for each of said first plurality of RTDs.